1. Field of the Invention
The present invention relates to a semiconductor photodetector and an avalanche photodiode, which are employed in a high-speed optical communication, etc.
2. Description of the Prior Art
A sectional structure of a semiconductor photodetector for the very high-speed optical communication in the prior art is shown in FIG. 1.
In the structure in FIG. 1, an N-type semiconductor layer 2 made of N-type InP, an optical waveguide core layer 3 made of undoped InGaAsP, and an upper cladding layer 4 made of undoped InP are formed in this sequence on a predetermined region of an undoped InP substrate 1, and these layers constitute an optical waveguide 5. Then, a photo absorbing layer 6 made of undoped InGaAs and a P-type semiconductor layer 7 made of P-type InP are formed in this sequence on the N-type semiconductor layer 2 located near an end portion of the optical waveguide 5, these layers as well as the N-type semiconductor layer 2 constitute a PIN photodiode 8.
In this semiconductor photodetector, in order to attain a sufficient quantum efficiency in the photo absorbing layer 6 whose thickness is thin, the light emitted from the optical waveguide core layer 3 is incident upon the photo absorbing layer 6 from the parallel direction to the jointed surface between the N-type semiconductor layer 2 and the photo absorbing layer 6. Assume that such structure is referred to as a “lateral-type incident structure” hereinafter. Such structure, although not disclosed, is set forth in Japanese Patent Application No.2002-214408.
Then, when the light is input in the state that a reverse bias voltage is applied to an N-side electrode 9 and a P-side electrode 10, pairs of electron-hole are produced in the photo absorbing layer 6 and thus a photo current is sensed.
In this case, as the photodiode, in addition to the above PIN photodiode, the avalanche photodiode (APD) set forth in Patent Literature 1 is also known. In addition, not the lateral-type incident structure but the planar-type avalanche photodiode having the guard ring structure is disclosed in Patent Literature 2, for example. Then, the operating characteristic of the avalanche photodiode is disclosed in Non-Patent Literature 1.
[Patent Literature 1]
    Patent Application Publication (KOKAI) Hei 11-354827[Patent Literature 2]    Patent Application Publication (KOKAI) Hei 10-209486[Non-Patent Literature 1]    R. B. Emmons, J. Appl. Phys. 38, 3705, 1967
In the semiconductor photodetector having the lateral-type incident structure, it is desired that the further large photo current should be extracted and also the receiving sensitivity should be enhanced. For this reason, it is preferable that the photodiode that is different from the PIN photodiode 8 should be employed.
Further, in such avalanche photodiode, it is preferable that not only simply the receiving sensitivity can be increased but also an operation at a high speed should be performed in a manner such that the light signal being modulated at a high speed can be received and then the photo current that follows up such light signal can be obtained.